Such a dragged fishing net is usually dragged by a vessel, such as for example a fishing boat.
In order to protect the fish stocks and to fish as efficiently as possible, only fish of a minimum desired longitudinal dimension and/or only fish of a certain species should be caught.
Until now, in so far as size is concerned, this has been achieved by means of modified constructions of the fishing net and fishing gear. These modified constructions ensure that fish of incorrect size can escape from the fishing net. Examples thereof are:                providing a fishing net of a certain mesh width size, so that smaller fish can escape;        providing panels containing larger square meshes in the fishing net, thus allowing fish species which tend to swim towards the surface to escape.        
However, these systems are not able to sort the fish in a completely reliable manner, as a smaller fish may not be able to swim through the mesh of the net due to, for example, fatigue and/or may become stuck between other bigger fish. After a certain fishing period, the fishing net is raised and brought on board of the fishing boat. On board, the non-commercial fish are sorted from the caught fish and optionally thrown overboard. Fish which have been thrown overboard may or may not survive and continue to grow.